High-impedance faults (HIFs) are difficult to detect on multi-grounded electrical power distribution systems. One reason for this difficulty is that many of the loads on the system are single-phase in nature. The system can therefore be quite unbalanced when a major single-phase lateral is out of service. The conventional ground fault protection has to be set above the maximum foreseeable unbalance to avoid false tripping. Conventional ground fault protection is thus not effective for HIF protection.
The second reason that HIFs are difficult to detect is that the HIF current level is typically small. For example, the HIF current level may range anywhere from about zero amperes to less than 100 amperes depending upon the ground surfaces that a power conductor contacts.
Many untraditional algorithms have been developed to detect HIF at the substation level in the power distribution system. Most of these algorithms use off-fundamental frequency components of phase or residual currents, and use complicated methods such as expert systems, neural networks, wavelets and higher-order statistics.
An algorithm that detects certain HIFs is relatively easy to design. However, it is a greater challenge to also design a secure algorithm. Electrical utilities desire to have secure HIF protection. The objective of HIF protection is not to protect the power system and apparatus from damage resulting from an HIF. Rather, the objective of implementing secure HIF protection is to remove an unsafe public condition, which may also avoid or minimize any attendant legal issues relating to the condition. When a detection device indicates the occurrence of an HIF, a utility has to make a decision based upon the circumstances. For example, it may be more dangerous to trip the electrical power to a traffic light at a busy traffic intersection or to a hospital. For such reasons, utilities cannot tolerate false alarms from HIF detection devices.
Moreover, when an HIF is detected, a number of factors or circumstances may need to be considered before a tripping decision is made. For example, it may be more dangerous to trip the power to traffic lights at a busy intersection, or to a hospital. For reasons such as these, a utility cannot normally tolerate any false HIF detection.
A general object of the present invention is to therefore provide improved systems and methods for detecting an HIF in a multi-grounded distribution system.
Another object of the present invention is to provide improved systems and methods for detecting an HIF that is secure from false detections.
A further object of the present invention is to provide improved systems and methods for detecting an HIF that may be easily incorporated into existing distribution relays and related equipment.